Conventionally, in an optical communication system, there has been used a direct modulation system that generates an optical modulation signal based on a driving current to a laser diode and obtains a light intensity signal that is proportional to an electric signal of the driving current. However, in a super-high-speed broadband optical communication system having a transmission speed that exceeds a few G bits/s, a chirping occurs in which an optical wavelength changes during a direct modulation. This chirping limits the transmission capacity.
In the mean time, according to the external modulation system, there occurs little chirping, and it is possible to modulate relatively easily in an operation band of 10 GHz or above. Therefore, the external modulation system has come to be applied to a super-high-speed broadband large-capacity optical communication system. One of the most general optical modulators as an external modulator, is a Mach-Zehnder optical modulator that uses lithiumniobate (LiNbO3).
An output optical signal I(t) obtained by modulation based on a modulation signal S(t) using this Mach-Zehnder optical modulator is expressed by the following equation (1).I(t)=k{1+cos (β*S(t)+δ)}  (1)where, “k” represents a proportional coefficient, “β” represent a modulation factor, and “δ” represents a phase of an operation point of a Mach-Zehnder optical modulator.
Assume that the modulation signal S(t) is a two-value digital signal, the modulation factor β is set as β=π, a suitable DC voltage (bias voltage) is applied to the Mach-Zehnder optical modulator, and an initial phase δ is set as δ=π/2. Then, the Mach-Zehnder optical modulator outputs the output optical signal I(t) that is turned ON/OFF in proportion to the modulation signal S(t).
Next, assume that the modulation factor β is set as β=2π, a suitable bias voltage is applied to the Mach-Zehnder optical modulator, and an initial phase δ is set as δ=0. Further, a sinusoidal wave of a repetitive frequency fc is input as the modulation signal S(t). Then, the output optical signal I(t) can be expressed by the following equation (2).I(t)=k{1+cos (2π*sin (2πfc(t}))  (2)Therefore, the output optical signal I(t) shown by the equation (2) is output as an optical signal that is turned ON/OFF in the repetitive frequency 2fc that is two times the repetitive frequency fc.
In this case, although there is no problem when the value of the phase δ is constant, an optical modulator that uses a normal lithium niobate has a problem in that the operation point drifts. This drift includes two types: a thermal drift due to a pyroelectric effect that is brought about by a temperature change; and a DC drift that is generated by a charge distribution that is formed on the surface of elements of the optical modulator by the bias voltage applied to the electrode of the optical modulator. In order to compensate for the change in the operation point due to these drifts, it is necessary to apply a bias voltage to the optical modulator to obtain an optimum operation point.
FIG. 12 is a block diagram showing a structure of a conventional optical transmission apparatus capable of stabilizing a bias voltage applied to the optical modulator that uses this lithium niobate (refer to Japanese Patent Application Laid-Open No. 5-142504). In FIG. 12, a continuous light emitted from a light source 101 is input into a Mach-Zehnder optical modulator 103 that uses lithium niobate. A terminating unit 114 is connected to the Mach-Zehnder optical modulator 103. Further, a driving signal for driving the Mach-Zehnder optical modulator 103 and a bias voltage are applied to the Mach-Zehnder optical modulator via a node T1.
An output optical signal modulatedby the Mach-Zehnder optical modulator 103 is output to an output terminal 120 via a branching filter 104. At the same time, a part of the output optical signal is input into a photodiode 105. The photodiode 105 converts the input part of the output optical signal into an electric signal, and a preamplifier 106 amplifies this electric signal, and outputs a result to a synchronous detector circuit 107.
The synchronous detector circuit 107 carries out a synchronous detection of the electric signal input from the preamplifier 106 and a low-frequency signal output from a dither signal generator 112. The synchronous detector circuit 107 has a mixer 117, and mixes the electric signal input from the preamplifier 106 and the low-frequency signal output from the dither signal generator 112. The mixed signal is input into a low-pass transmission filter 109 via an operational amplifier 108. The low-pass transmitted signal is output to a bias voltage control circuit 110.
The bias voltage control circuit 110 has a DC power source 118 and an adder 119. The adder 119 adds a signal output from the synchronous detector circuit 107 and a bias voltage output from the DC power source 118 together, and outputs an added result as a bias voltage to the Mach-Zehnder optical modulator 103 from the node T1 via an inductor 111. On the other hand, a driving signal is input from an input terminal 121, and is output to a low-frequency superimposing circuit 113 via a driving circuit 124. The low-frequency superimposing circuit 113 superimposes the input driving signal and the low-frequency signal output from the dither signal generator 112 together, and applies a superimposed result as a driving signal to the Mach-Zehnder optical modulator 103 from the node T1 via a capacitor. Therefore, both the driving signal that has been superimposed with the low-frequency signal and the bias voltage that has been bias-voltage controlled are applied to the Mach-Zehnder optical modulator 103 from the node T1.
A method of bias-voltage controlling the Mach-Zehnder optical modulator according to a conventional optical transmission apparatus will be explained below with reference to FIG. 13 to FIG. 15. FIG. 13 is a diagram for explaining a modulation operation of the Mach-Zehnder optical modulator 103 when the bias voltage (phase δ) is at a proper value. In FIG. 13, a performance characteristic curve 130 of the Mach-Zehnder optical modulator 103 shows a performance characteristic curve shown in the equation (1). This shows a state that the bias voltage (phase δ) has been set to a proper value. In this case, when a driving signal (an input signal) 131 superimposed with a low-frequency signal has been input into the Mach-Zehnder optical modulator 103, the driving signal 131 is modulated by the performance characteristic curve 130, and the modulated signal is output as an output optical signal 132. This output optical signal 132 does not include a low-frequency component (f [Hz]) of the low-frequency signal superimposed with the driving signal. A low-frequency component (2 f [Hz]) that is two times the low-frequency component (f [Hz]) is generated in this output optical signal 132. Therefore, the photodiode 105 receives a part of the output optical signal 132, and the preamplifier 106 amplifies this result. Thereafter, the synchronous detector circuit 107 carries out a synchronous detection. As a result, the output of the signal becomes “0”. In this case, there is no signal component to be added by the adder 119 of the bias voltage control circuit 110. Therefore, the current bias voltage is maintained as it is, and this bias voltage is applied to the Mach-Zehnder optical modulator 103.
On the other hand, FIG. 14 is a diagram for explaining a modulation operation of the Mach-Zehnder optical modulator 103 when the bias voltage is at a value slightly higher than a proper value. In FIG. 14, a performance characteristic curve 140 of the Mach-Zehnder optical modulator 103 shows a state that the bias voltage has been set to a value slightly higher than a proper value. In this case, when a driving signal 141 that is the same as the driving signal 131 superimposed with a low-frequency signal has been input into the Mach-Zehnder optical modulator 103, the driving signal 141 is modulated by the performance characteristic curve 140, and the modulated signal is output as an output optical signal 142. This output optical signal 142 includes a low-frequency component (f [Hz]) of the low-frequency signal superimposed with the driving signal. The phase of this low-frequency component (f [Hz]) has been inverted from the phase of the low-frequency component (f [Hz]) that has been superimposed with the driving signal. Therefore, the synchronous detector circuit 107 carries out a synchronous detection of the low-frequency component (f [Hz]), and outputs a result to the bias voltage control circuit 110 as a “negative” voltage. In this case, the adder 119 of the bias voltage control circuit 110 adds the negative voltage to the bias voltage output from the DC power source 118, and controls the current bias voltage to become close to the proper value of the bias voltage, by making the current bias voltage smaller.
Further, FIG. 15 is a diagram for explaining a modulation operation of the Mach-Zehnder optical modulator 103 when the bias voltage is at a value slightly lower than a proper value. In FIG. 15, a performance characteristic curve 150 of the Mach-Zehnder optical modulator 103 shows a state that the bias voltage has been set to a value slightly lower than a proper value. In this case, when a driving signal 151 that is the same as the driving signal 131 superimposed with a low-frequency signal has been input into the Mach-Zehnder optical modulator 103, the driving signal 151 is modulated by the performance characteristic curve 150, and the modulated signal is output as an output optical signal 152. This output optical signal 152 includes a low-frequency component (f [Hz]) of the low-frequency signal superimposed with the driving signal. The phase of this low-frequency component (f [Hz]) coincides with the phase of the low-frequency component (f [Hz]) that has been superimposed with the driving signal. Therefore, the synchronous detector circuit 107 carries out a synchronous detection of the low-frequency component (f [Hz]), and outputs a result to the bias voltage control circuit 110 as a “positive” voltage. In this case, the adder 119 of the bias voltage control circuit 110 adds the positive voltage to the bias voltage output from the DC power source 118, and controls the current bias voltage to become close to the proper value of the bias voltage, by making the current bias voltage larger.
As explained above, according to the bias voltage control for controlling a bias voltage applied to the Mach-Zehnder optical modulator of the conventional optical transmission apparatus, a part of the output optical signal output from the Mach-Zehnder optical modulator 103 is detected. The synchronous detector circuit 107 generates an error signal corresponding to a deviation of the bias voltage from an optimum operation point. The bias voltage control circuit 110 controls the bias voltage so that this error signal becomes smaller, thereby maintaining a stable bias voltage.
According to the bias voltage control for controlling a bias voltage applied to the Mach-Zehnder optical modulator 103 of the conventional optical transmission apparatus, a low-frequency signal is superimposed with the driving signal. However, the low-frequency superimposing circuit 113 for superimposing this low-frequency signal with the driving signal uses devices like a voltage control attenuator and a voltage control variable gain amplifier not shown. Therefore, when the band of the driving signal becomes 10 GHz or above, the operation band for these devices becomes in shortage, and a waveform distortion is generated in the driving signal to be applied to the Mach-Zehnder optical modulator 103. As a result, there has been a problem of an occurrence of quality degradation in the output optical signal.
Further, when the band of the driving signal becomes 20 GHz or above, the operation band of the driving circuit 124 becomes in shortage, and thus the driving circuit 124 generates a waveform distortion in the driving signal. As a result, there has been a problem of an occurrence of quality degradation in the output optical signal.
The conventional optical transmission apparatus has obtained an optical output that is proportional to the repetitive frequency fc of the driving signal. However, it has been desired that it is also possible to control the stability of the bias voltage of the Mach-Zehnder optical modulator for an optical transmission apparatus that outputs an output optical signal having the repetitive frequency 2 fc that is two times the repetitive frequency fc of the driving signal.